Scintillators are substances which absorb radiations such as γ-rays or X-rays and emit visible light or electromagnetic waves of wavelengths near those of visible light. Examples of their applications include various types of radiation detectors such as medical PET (positron emission tomography equipment) and TOF-PET (time-of-flight positron emission tomography equipment), X-ray CT (X-ray computed tomography equipment), belongings inspection equipment used in airports and the like, cargo inspection equipment used in harbors and the like, oil search equipment, further exposure dose measurement equipment and high-energy particle measurement equipment.
Such radiation detectors are generally constituted of a scintillator section to receive radiations and convert it to visible light, and a light detection section, such as a photomultiplier tube (hereinafter, referred to as “PMT”) or a photodiode, to detect the visible light converted in and transmitted from the scintillator section and convert it to electric signals. Then, scintillators used in such applications are, in order to make noises low and measurement precision high, desired to be ones high in the luminescent output.
As scintillators, alkali halide crystals such as CsI and NaI are conventionally broadly put into practical use. Among these, a scintillator having CsI as a host material thereof is utilized from the point that the radiation absorption efficiency is relatively high, the point that the radiation damage is relatively small, the point that the thin film fabrication is relatively easy by a vacuum deposition method or the like, and other points.
However, since the luminescent efficiency of conventional CsI scintillators is not very high, CsI scintillators in which a crystal having CsI as a host material thereof is doped with impurities to enhance the scintillation efficiency, and CsI:Na, CsI:Tl and the like in which TlI (thallium iodide) is doped are put into practical use.
For example, Japanese Patent Laid-Open No. 2008-215951 discloses a cesium iodide:thallium (CsI:Tl) in which cesium iodide is doped with thallium (Tl).
Further International Publication No. WO 2013/027671 discloses a scintillator improved in the afterglow characteristics, in which a crystal material containing CsI (cesium iodide) as a host material thereof and thallium (Tl) as a luminescent center thereof is doped with bismuth (Bi).
As a result of studies carried out on a scintillator disclosed in the above International Publication No. WO 2013/027671, that is, on a scintillator having a crystal containing CsI (cesium iodide) as a host material thereof and thallium (Tl) and bismuth (Bi), it has been made clear that by regulating the concentration of bismuth (Bi) in a predetermined low range, the output is maintained, and the afterglow characteristics can be simultaneously improved. It has also been made clear, however, that only by simply making the concentration of bismuth (Bi) low, it is difficult to maintain the high output and simultaneously further enhance the afterglow characteristics.